When a hard disk is manufactured a special pattern is written in a code called a Gray code on the surface of the platters, while the drive is open in a clean room, with a machine called a servo writer. Conventional servo writing is performed in a clean room environment with external sensors invading a disk-drive head disk assembly to provide the precise angular and radial position information to write the servo patterns on the disk.
There are a fixed number of servo sectors per track and the sectors are adjacent to one another. This pattern is permanent and cannot be changed by writing normal data to the drive.
The electronics use feedback from the heads, which read the Gray code pattern, to very accurately position, and constantly correct the radial position of the appropriate head over the desired track, at the beginning of each sector, to compensate for variations in platter geometry, caused by mechanical stress and thermal expansion and contraction. Altogether, the head positioning components form what is known as closed-loop servo system—a marvelous (and, perhaps, dangerous) thing to watch operate in a drive which has been opened.
Recently, tools have been developed that write the servo pattern on multiple disks prior to the assembly of the drive head. This process can increase the rate at which servo patterns are written, however the tolerances need to be very small between the center of the servo pattern and physical center of the disk in order to enable a subsequently attached thin film disk head to be able to read and write onto the thin film magnetic disk.
What is needed is an automatic way of measuring the servo pattern and physical disk to identify a difference between the center of a servo pattern and the center of the physical disk in order to keep the distance between the two centers within a specified tolerance.